1. Field of the Invention
The present invention relates to an Optical Code Division Multiplexing (OCDM) communication system, and more particularly to a method for detecting and correcting a failure that occurs when transmission information is encoded and decoded and an OCDM communication system having functions to implement the method.
2. Description of the Related Art
Recently, demand for communication has rapidly increased along with spread of the Internet or the like and a network having large capacity and high speed using optical fibers has been implemented to meet such demand. An optical multiplexing technology which collectively transmits optical pulse signals of multiple channels through one optical fiber has been regarded as important to increase communication capacity.
Optical Code Division Multiplexing (OCDM) is under study as an optical multiplexing technology. OCDM can save communication resources since one channel does not occupy physical resources such as a time slot or wavelength. OCDM can also achieve high speed processing since a passive element can be used as a means for encoding and decoding transmission information and thus a series of processes from encoding to decoding can be performed using optical signals without conversion. In addition, OCDM does not require synchronization between channels and thus can simplify system configuration.
In a communication system using the OCDM scheme, some devices use, for example, a Waveguide-Grating-Router (WGR) described in Non-Patent Reference 1, which is described later, or a Superstructured Fiber Bragg Grating (SSFBG) described in Non-Patent Reference 2, which is described later, as a passive element for encoding and decoding transmission information.
The principle of OCDM described in Non-Patent Reference 2 is described as follows. FIG. 1 illustrates details of a process for encoding an input signal using an SSFBG, which is a passive element, as an encoding means. When a pulsed input signal 11 is input, the input signal 11 is input to an SSFBG 13 through a circulator 12. The SSFBG 13 includes N unit gratings 131, 132, . . . , 13N connected in cascade. Each of the unit gratings partially transmits and partially reflects light having the same wavelength as the Bragg reflection wavelength. Since the unit gratings are present in the SSFBG 13 at different positions, the input signal 11 is reflected and re-coupled at the N positions in the SSFBG 13 in the case where all unit gratings 131 and 13N have the same Bragg reflection wavelength and the wavelength of the input signal 11 is equal to the Bragg reflection wavelength of the unit gratings. Here, let 14i be light reflected at a specific unit grating 13i, where i is one of 1 to N. This reflected light 14i is referred to as a “chip pulse”. N corresponding to the number of unit gratings in the SSFBG 13 is referred to as the “number of chips” or “chip count”. Chip pulses are output through the circulator 12, thereby obtaining an output signal 15 including the N chip pulses.
Let Lj be the distance between adjacent unit gratings 13j and 13j+1, where j is one of 1 to N−1. Here, since a time interval Tcj between the chip pulses 14j and 14j+1 is equal to a round-trip propagation time between adjacent unit gratings 13j and 13j+1, the time interval Tcj is expressed as follows.
                              Tc          j                =                                            2              ⁢                              n                eff                            ⁢                              L                j                                      c                    ❘                                    (        1        )            
Here, c is the speed of light in a vacuum, and neff is an effective index of refraction of the SSFBG.
A phase difference θj between the chip pulses 14j and 14j+1 is expressed as follows.
                              θ          j                =                                                            4                ⁢                π                ⁢                                                                  ⁢                                  n                  eff                                ⁢                                  L                  j                                                            λ                0                                      +                          2              ⁢              π              ⁢                                                          ⁢              n                                ❘                                    (        2        )            
Here, λ0 is the optical wavelength of the signal (i.e., the Bragg reflection wavelength of the unit grating) and n is an arbitrary integer. This phase difference θj is an encoding pattern used for encoding the input signal.
Signals multiplexed through encoding and decoding processes can be identified using both the process for dividing one pulse into a plurality of chip pulses as described above and phase difference information between the chip pulses. This encoding scheme is referred to as a “time spreading scheme” since one pulse is divided into a plurality of pulses and the pulses are arranged on the time axis. This encoding scheme is also referred to as a “coherent time spreading scheme” since the encoding scheme uses phase information.
The operating principle of OCDM using coherent time spreading encoding is described as follows with reference to FIGS. 2A to 2D. FIG. 2A is a block diagram illustrating a schematic configuration of a communication system that performs communication using the OCDM scheme, FIG. 2B illustrates a waveform of the intensity of an optical output signal S1 transmitted by a transmitter 21, FIG. 2C illustrates a waveform of the intensity of an encoded signal S2 transmitted by an encoder 22, and FIG. 2D illustrates a waveform of the intensity of a decoded signal S3 transmitted by a decoder 23.
The transmitter 21 is a means for transmitting digital information and transmits 1-bit information at regular intervals. The 1-bit information transmitted by the transmitter 21 is represented by one optical pulse. For example, data “1” is represented by presence of an optical pulse and data “0” is represented by absence of an optical pulse. The optical output signal S1 from the transmitter 21 is provided to the encoder 22.
The encoder 22 includes an SSFBG having a plurality of unit gratings. The encoder 22 encodes the optical output signal S1 provided from the transmitter 21 according to a code value which is based on the structure of the SSFBG and outputs an encoded signal S2 including a number of chip pulses corresponding to the number of chips in the SSFBG, and then provides the encoded signal S2 to the decoder 23.
The decoder 23 includes an SSFBG having the same structure as that of the encoder 22. The decoder 23 again divides each of the chip pulses generated by the encoder 22 into a number of pulses corresponding to the number of chips of the SSFBG of the decoder 23. Each of the chip pulses generated through division at the decoder 23 is referred to as a “divided chip pulse”. That is, the decoder 23 generates a number of divided chip pulses corresponding to the product of the number of chips of the encoder 22 and the number of chips of the decoder 23. Each of the divided chip pulses has a phase corresponding to the code value of the decoder 23. The decoder 23 adds the divided chip pulses having different phases taking into consideration the time intervals and phases of the divided chip pulses and outputs and provides the resulting signal as a decoded signal S3 to the receiver 24. Here, the peak intensity of the decoded signal S3 is high when the encoder 22 and the decoder 23 have the same code value. On the other hand, the peak intensity of the decoded signal S3 is low when the encoder 22 and the decoder 23 have different code values. The receiver 24 receives the decoded signal S3 provided by the decoder 23 and reproduces digital information transmitted by the transmitter 21. The receiver 24 can reproduce the digital information only when the decoded signal S3 provided from the decoder 23 has a high peak intensity, i.e., only when the code values of the encoder 22 and the decoder 23 are equal. OCDM accomplishes multiplexed transmission of optical signals by allocating a different code value to each channel.
If code values are created according to the above Equations (1) and (2), phase differences θj between chip pulses are different and therefore distances between unit gratings Lj are also different and thus time intervals Tcj between adjacent chip pulses are also different. However, when mathematical models are created assuming that all time intervals Tcj are equal, this assumption has almost no influence upon OCDM operation for the reason described below. Therefore, in the following description, it is assumed that all time intervals between adjacent pulses are equal to “Tc” for the sake of ease of explanation.
Changing of the phase difference θj between adjacent chip pulses from 0 to 2π is sufficient for encoding. Lj required to change θj by 2π according to Equation (2) is on the order of the signal wavelength since the effective index of refraction neff of the SSFBG is generally about 1.5. A corresponding time interval is on the order of the reciprocal of the optical frequency of the signal. Generally, the time width of light generated from a pulse light source is sufficiently larger than the reciprocal of the optical frequency. Therefore, even though time intervals between adjacent chip pulses are fixed to the specific value Tc, errors of the time intervals are sufficiently smaller than the chip pulse width and therefore it can be assumed that there is very little change in overlapping between the chip pulses generated through division in the decoding operation described above.
The operation of the encoder 22 is similar to that described above with reference to FIG. 1. Phase differences between adjacent ones of the chip pulses generated by the encoder 22 are denoted by θe1, θe2, . . . θeN−1 in the order of time. When the electric field time waveform of the input pulse is represented by p(t) (t: time), an electric field time waveform E(t) of the encoded light is obtained as follows.
                                                                        E                ⁡                                  (                  t                  )                                            =                            ⁢                                                                    1                                          N                                                        ⁢                                      p                    ⁡                                          (                      t                      )                                                                      +                                                      1                                          N                                                        ⁢                                      p                    ⁡                                          (                                              t                        -                        Tc                                            )                                                        ⁢                                      exp                    ⁡                                          [                                              ⅈθ                        ⁢                                                                                                  ⁢                                                  e                          1                                                                    ]                                                                      +                                                                                                      ⁢                                                                    1                                          N                                                        ⁢                                      p                    ⁡                                          (                                              t                        -                                                  2                          ⁢                          Tc                                                                    )                                                        ⁢                                      exp                    ⁡                                          [                                              ⅈθ                        ⁢                                                                                                  ⁢                                                  e                          1                                                                    ]                                                        ⁢                                      exp                    ⁡                                          [                                              ⅈθ                        ⁢                                                                                                  ⁢                                                  e                          2                                                                    ]                                                                      +                …                +                                                                                                      ⁢                                                1                                      N                                                  ⁢                                  p                  ⁡                                      (                                          t                      -                                                                        (                                                      N                            -                            1                                                    )                                                ⁢                        Tc                                                              )                                                  ⁢                                  exp                  ⁡                                      [                                          ⅈθ                      ⁢                                                                                          ⁢                                              e                        1                                                              ]                                                  ⁢                                  exp                  ⁡                                      [                                          ⅈθ                      ⁢                                                                                          ⁢                                              e                        2                                                              ]                                                  ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  exp                  ⁡                                      [                                          ⅈθ                      ⁢                                                                                          ⁢                                              e                                                  N                          -                          1                                                                                      ]                                                                                                                          =                            ⁢                                                1                                      N                                                  ⁢                                  {                                                            p                      ⁡                                              (                        t                        )                                                              +                                                                  ∑                                                  j                          =                          1                                                                          N                          -                          1                                                                    ⁢                                                                        p                          ⁡                                                      (                                                          t                              -                                                              j                                ⁢                                                                                                                                  ⁢                                Tc                                                                                      )                                                                          ⁢                                                  exp                          ⁡                                                      [                                                          ⅈ                              ⁢                                                                                                ∑                                                                      k                                    =                                    1                                                                    j                                                                ⁢                                                                  θ                                  ⁢                                                                                                                                          ⁢                                                                      e                                    k                                                                                                                                                        ]                                                                                                                                }                                                                                        (        3        )            
Here, it is assumed that the magnitudes of the amplitudes of all chip pulses are equal and the magnitudes thereof are expressed in an arbitrary scale. In Equation (3), “i” represents the imaginary unit.
When the receiver 24 receives one chip pulse generated by the encoder 22, the receiver 24 divides the chip pulse into chip pulses as described above. Phase differences between adjacent ones of the divided chip pulses are represented by θd1, θd2, . . . θdN−1. Here, the electric field time waveform D(t) of the decoded light is expressed as follows.
                                                                                                                               D                    ⁡                                          (                      t                      )                                                        =                                    ⁢                                                                                    1                                                  N                                                                    ⁢                                              E                        ⁡                                                  (                          t                          )                                                                                      +                                                                  1                                                  N                                                                    ⁢                                              E                        ⁡                                                  (                                                      t                            -                            Tc                                                    )                                                                    ⁢                                              exp                        ⁡                                                  [                                                      ⅈθ                            ⁢                                                                                                                  ⁢                                                          d                              1                                                                                ]                                                                                      +                                                                                                                                          ⁢                                                                            1                                              N                                                              ⁢                                          p                      ⁡                                              (                                                  t                          -                                                      2                            ⁢                            Tc                                                                          )                                                              ⁢                                          exp                      ⁡                                              [                                                  ⅈθ                          ⁢                                                                                                          ⁢                                                      d                            1                                                                          ]                                                              ⁢                                          exp                      ⁡                                              [                                                  ⅈθ                          ⁢                                                                                                          ⁢                                                      d                            2                                                                          ]                                                                              +                  …                  +                                                                                                                        ⁢                                                      1                                          N                                                        ⁢                                      E                    ⁡                                          (                                                                        t                          ⁡                                                      (                                                          N                              -                              1                                                        )                                                                          ⁢                        Tc                                            )                                                        ⁢                                      exp                    ⁡                                          [                                              ⅈθ                        ⁢                                                                                                  ⁢                                                  d                          1                                                                    ]                                                        ⁢                                      exp                    ⁡                                          [                                              ⅈθ                        ⁢                                                                                                  ⁢                                                  d                          2                                                                    ]                                                        ⁢                                                                          ⁢                  …                  ⁢                                                                          ⁢                                      exp                    ⁡                                          [                                              ⅈθ                        ⁢                                                                                                  ⁢                                                  d                                                      N                            -                            1                                                                                              ]                                                                                                                                              =                                ⁢                                                      1                                          N                                                        ⁢                                      {                                                                  E                        ⁡                                                  (                          t                          )                                                                    +                                                                        ∑                                                      j                            =                            1                                                                                N                            -                            1                                                                          ⁢                                                                              p                            ⁡                                                          (                                                              t                                -                                                                  j                                  ⁢                                                                                                                                          ⁢                                  Tc                                                                                            )                                                                                ⁢                                                      exp                            ⁡                                                          [                                                              ⅈ                                ⁢                                                                                                      ∑                                                                          k                                      =                                      1                                                                        j                                                                    ⁢                                                                      θ                                    ⁢                                                                                                                                                  ⁢                                                                          d                                      k                                                                                                                                                                  ]                                                                                                                                            }                                                                                                                   (        4        )            
Here, it is assumed that the magnitudes of the amplitudes of all divided chip pulses are equal and the magnitudes thereof are expressed in an arbitrary scale. From Equation (4), it can be understood that the time waveform of the decoded optical signal has the following structure. That is, the time waveform of the decoded optical signal includes one divided chip pulse at the beginning of the time waveform as shown in FIG. 2D. When I1 is an integer which is equal to or greater than 1 and equal to or less than N−1, a pulse generated through superposition of I3+1 divided chip pulses having different phases appears after a time of I1×Tc from the beginning divided chip pulse. When I2 is an integer which is equal to or greater than N and equal to or less than 2N−3, a pulse generated through superposition of 2N−I2−1 divided chip pulses having different phases appears after I2×Tc from the beginning divided chip pulse. One pulse appears after 2(N−1)×Tc from the beginning divided chip pulse. Here, taking into consideration the signal intensity after (N−1)×Tc from the beginning divided chip pulse, at the moment when N−1 conditions expressed by Equation (5) are satisfied simultaneously, the phases of the N divided chip pulses are all equal according to Equations (3) and (4) and the intensity of the resulting pulse is N2 times as high as the intensity of one divided chip pulse.θej=0dN−j(j=1,2, . . . N−1)|  (5)
At other times, N−1 divided chip pulses exhibit maximal overlap. When the conditions of Equation 5 are not satisfied, a pulse whose intensity is N2 times as high as the intensity of one divided chip pulse cannot be obtained at any time. Accordingly, the intensity is the peak intensity from among intensities obtained through encoding using all combinations of code values. Therefore, when code values satisfying the conditions of Equation (5) are assigned to the encoder and the decoder, it is assumed that the code values are equal and other combinations of code values are different.
An encoding scheme that identifies a received optical signal obtained by assigning different code values to the transmitter and the receiver and a received optical signal obtained by assigning the same code value to the transmitter and the receiver while satisfying the conditions of Equation 5 has been suggested in Non-Patent Reference 3 described later. In the encoding scheme described in Non-Patent Reference 3, phase differences between adjacent chip pulses are all equal and the phase difference is used as a code value. This encoding scheme is employed in encoding and decoding devices described in Non-Patent References 1 and 2.
In the case where all phase differences between adjacent chip pulses are equal to θe when encoding is performed and all phase differences between adjacent divided chip pulses are equal to θd when decoding is performed, an electric field time waveform E′(t) of encoded light is expressed as follows according to the definition of the code value and Equation (6).
                                          E            ′                    ⁡                      (            t            )                          =                              1                          N                                ⁢                      {                                          p                ⁡                                  (                  t                  )                                            +                                                ∑                                      j                    =                    1                                                        N                    -                    1                                                  ⁢                                                      p                    ⁡                                          (                                              t                        -                                                  j                          ⁢                                                                                                          ⁢                          Tc                                                                    )                                                        ⁢                                      exp                    ⁡                                          [                                              ⅈ                        ⁢                                                                                                  ⁢                        j                        ⁢                                                                                                  ⁢                        θ                        ⁢                                                                                                  ⁢                        e                                            ]                                                                                            }                                              (        6        )            
An electric field time waveform D′(t) of decoded light is expressed as follows according to the definition of the code value and Equation (7).
                                          D            ′                    ⁡                      (            t            )                          =                              1                          N                                ⁢                      {                                                            E                  ′                                ⁡                                  (                  t                  )                                            +                                                ∑                                      j                    =                    1                                                        N                    -                    1                                                  ⁢                                                                            E                      ′                                        ⁡                                          (                                              t                        -                                                  j                          ⁢                                                                                                          ⁢                          Tc                                                                    )                                                        ⁢                                      exp                    ⁡                                          [                                              ⅈ                        ⁢                                                                                                  ⁢                        j                        ⁢                                                                                                  ⁢                        θ                        ⁢                                                                                                  ⁢                        d                                            ]                                                                                            }                                              (        7        )            
Non-Patent Reference 1: G. Cincotti, et al., “Characterization of Full Encoder/Decoder in the AWG Configuration for Code-Based Photonic Routers—Part I: Modeling and Design,” Journal of Lightwave Technology Vo. 24, No. 1, pp. 103-112, January 2006
Non-Patent Reference 2: Kobayashi, et al., “Experimental Study on Code Reconfigurability in FBG Based Encoder/Decoder”, IEICE Tech. Rep., OPE2008-86, pp. 127-132, August 2008.
Non-Patent Reference 3: G. Cincotti, et al., “Full Optical Encoders/Decoders for Photonic IP Routers,” Journal of Lightwave Technology Vo. 22, No. 2, pp. 337-342, February 2004
In the OCDM communication system including the encoder and the decoder, there is a need to maintain the code value set in the encoder and the decoder during communication. That is, in the case where the phase difference between adjacent chip pulses is used as the code value, the receiver may fail to reproduce original digital information when the phase difference has changed. The code value set in the encoder and the decoder changes when the optical path length of each optical waveguide included in the encoder and the decoder changes. This optical path length changes depending on stress such as tension applied to the optical waveguide or change of temperature of the optical waveguide. Therefore, during communication, there is a need to perform control for keeping the optical path length constant so as to prevent change in the code value set in the encoder and the decoder.
In OCDM communication, optical signals of multiple channels are multiplexed and transmitted in one optical fiber transmission path and the multiplexed optical signals are collectively received by the receiving side. In the case where a code value set in an encoder or decoder of one of the channels has changed, there is a need to specify (or identify) the channel of the encoder or decoder whose code value has changed by monitoring the optical signals received by the receiving side. In addition, there is a need to control the code value of the encoder or decoder of the specified channel so as to reach the original code value while minimizing influence exerted upon communication states of the other channels.